When gae_memcache passed expiration time to memcache Client
(see commit 59290534bc),
logic is no longer needed to calculate time delta, since memcache
handles that for us. Also, time_expire=0 was expiring values
immediately, instead of the documented behavior where 0 signified
no expiration.
Kept timestamp in value for backwards compatibility (is this
necessary since cached data is transient?)